The ABO Blood Groups
 
  Genotype     Antigen     Phenotype  
 
 
  IAIA     A     A
  IAIO     A  
 
  IBIB     B     B
  IBIO     B  
 
  IAIB     A, B     AB
 
  IOIO     Neither     O
 
The human ABO blood group is affected by multiple alleles.

An individual is A, B, AB, or O phenotype as a result of dominance of the IA and IB alleles to the IO allele and codominance of the IA and IB alleles to each other.